In certain types of work-load systems wherein the system is advanced through progressive or continuous displacements, forces are generated in opposition to the displacement itself. Generally, an input force is required to operate on the system so as to effect a displacement of the system or a component thereof. Frictional forces which are always present at least to some degree are not germane to the system of major forces being treated and therefore will be set aside as parasitic effects.
One example of a work-load system is the movement of an elevator panel on an airplane wing. As the elevator is raised from its normally flat position relative to the surface of the wing, the resistance offered by the flowing air mass increases with increasing displacement of the elevator. This is so since the reactive or resistance forces are proportional to the total cross-sectional surface area of the elevator presented against the flowing air mass. Frequently, the reactive forces generated on elevators are so great that hydraulic systems are necessary just to aid in the displacement of the elevator. However, even with force enabling hydraulic systems, the work output required increases with increasing displacement. During the return stroke, however, the stored energy is dissipated in a braking effort.
Since any reactive force can be represented by a spring or combinations thereof which upon displacement from their neutral or uncompressed states offer opposition to increased displacement, the discussion herein will be directed to spring systems by way of example and not limitation. In such cases, the greater the displacement, the greater the reactive force. Using the spring analogy the work-load system is said to have a positive spring "rate."
There are known devices which are intended to produce constant energy outputs. One such device is disclosed in U.S. Pat. No. 3,646,832 which relates to an energy control apparatus for automatically controlling the elastic energy of a coiled spring so that its resilience remains substantially constant. The embodiment as illustrated in FIG. 1 therein includes an operation lever pivotally supported on a shaft and having a segmental gear which meshes with a segmental gear on a pivotal member. A coiled spring is mounted on the shaft. The spring has one end portion engaging an eccentric cam on a lever and the other end portion engaging a roller mounted on the pivotal member. As the operation lever is moved, the angle .phi. formed by the two end portions varies. However, the difference between the angle .phi. before displacement of the lever and after is statedly smaller than in prior art arrangements. Hence, no significant increase is caused in the resilience of the spring. In the embodiment shown in FIG. 2, therein the spring has one end engaging a pin on a lever and the other end engaging a pivotal member pivotally supported by a shaft on which the lever is also pivotally supported. The pivotal member has a first segmental gear which meshes with a second segmental gear on a gear member. The latter also has a third segmental gear meshing with a fourth segmental gear on the lever. The ratio of the radius of rotation of the fourth gear to that of the third gear differs from the ratio for the first and second gears. Thus, proper selection of the values of the two ratios allegedly provides that the resilience of the spring will undergo substantially no change despite pivotal movement of the lever.
Notwithstanding the improvement offered by the apparatus of U.S. Pat. No. 3,646,832, this apparatus does not provide for energy storage and retrival during a typical cycle of a work-load system. Also, no recovery forces are obtainable to counterbalance forces generated by a work-load system so as to permit placement of the work-load system in a stationary position relative to the neutral position and to permit relative ease of continued displacement. For this reason, the system once displaced will not remain stationary in the displaced state but rather will tend to return to a former displacement.
Other spring systems which provide a constant force output include the Neg'ator spring manufactured by the Hunter Spring Co. and disclosed in U.S. Pat. Nos. 2,609,191; 2,609,192; 2,609,193; and 2,647,743. The Neg'ator extension spring is a roll of formed strip spring material which exerts a substantially constant restraining force to resist uncoiling. Although the Neg'ator spring is useful when employed to offset "constant force" work load systems, such as, for example, a gravitational system, it would not be suited to a work load system in which the load increases with displacement from a given starting position. Moreover, the Neg'ator is limited in size and material and is not generally applicable to large force systems.
In both spring applications described above an essentially zero spring rate has been achieved, meaning that the force delivered in opposing a work load system remains essentially constant throughout the displacement of the system.
Applying either a zero spring rate or a conventional positive spring rate arrangements to systems such as in aircraft controls, marine steering, etc, where the work loads increase with displacement (e.g., linearly with increased displacement) would only tend to compound the problem since additional input forces would be required with no savings in the net energy input.